linux-kernel - Re: [PATCH 2/3] x86: Define _PAGE_NUMA with unused physical address bits PMD and PTE levels

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Date:	Mon, 07 Apr 2014 10:37:55 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Mel Gorman <mgorman@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Cyrill Gorcunov <gorcunov@...il.com>, Peter Anvin <hpa@...or.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Noonan <steven@...inklabs.net>,
	Rik van Riel <riel@...hat.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Linux-MM <linux-mm@...ck.org>, Linux-X86 <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] x86: Define _PAGE_NUMA with unused physical address
 bits PMD and PTE levels

On 04/07/2014 08:10 AM, Mel Gorman wrote:
> +/*
> + * Software bits ignored by the page table walker
> + * At the time of writing, different levels have bits that are ignored. Due
> + * to physical address limitations, bits 52:62 should be ignored for the PMD
> + * and PTE levels and are available for use by software. Be aware that this
> + * may change if the physical address space expands.
> + */
> +#define _PAGE_BIT_NUMA		62

Doesn't moving it up to the high bits break pte_modify()'s assumptions?
 I was thinking of this nugget from change_pte_range():

	ptent = ptep_modify_prot_start(mm, addr, pte);
        if (pte_numa(ptent))
        	ptent = pte_mknonnuma(ptent);
	ptent = pte_modify(ptent, newprot);

pte_modify() pulls off all the high bits out of 'ptent' and only adds
them back if they're in newprot (which as far as I can tell comes from
the VMA).  So I _think_ it'll axe the _PAGE_NUMA out of 'ptent'.
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